Imaging medium

ABSTRACT

Acid can be generated by exposing a superacid precursor to actinic radiation effective to generate superacid from the superacid precursor and heating the superacid in admixture with a secondary acid generator capable of undergoing thermal decomposition to produce a secondary acid. The superacid catalyzes decomposition of the secondary acid generator, thus increasing the quantity of strong acid present in the medium. The resultant secondary acid can be used to effect a color change in an acid-sensitive material, so providing an imaging process.

REFERENCE TO PARENT APPLICATION

This application is a divisional of application Ser. No. 08/141,860,filed Oct. 22, 1993, (now U.S. Pat. No. 5,395,736) which in turn is acontinuation-in-part of our application Ser. No. 07/965,162, filed Oct.23, 1992 (now U.S. Pat. No. 5,334,489).

REFERENCE TO RELATED APPLICATIONS

Attention is directed to application Ser. No. 07/965,172 (now U.S. Pat.No. 5,278,031), and its divisional application Ser. No. 08/106,353,filed Aug. 13, 1993; these two applications describe and claim a processand imaging medium generally similar to those of the present invention,but in which the breakdown of a squaric acid derivative is initiatedthermally.

Attention is also directed to copending application Ser. No. 07/965,161(now U.S. Pat. No. 5,286,612) and its continuation-in-part, applicationSer. No. 08/141,852 of even date herewith and assigned to the sameassignee as the present application; these two applications describe andclaim a process and imaging medium generally similar to those of thepresent invention but in which acid is generated using a mixture of aninfra-red dye, a superacid precursor and an acid-sensitive acidgenerator. This mixture is exposed to an imagewise exposure to infra-redradiation, followed by a blanket exposure to ultra-violet radiation.

Finally, attention is directed to copending application Ser. No.08/084,759, filed Sep. 17, 1993 and assigned to the same assignee as thepresent application; this application describes and claims a process andimaging medium using a mixture of a diaryl iodonium salt and asquarylium dye capable of absorbing infra-red radiation having awavelength within the range of about 700 to about 1200 nm, the dyehaving a squarylium ring the 1- and 3-positions of which are eachconnected, via a single sp² carbon atom, to a pyrylium, thiopyrylium,benzpyrylium or benzthiopyrylium moiety, at least one of the sp² carbonatoms having a hydrogen atom attached thereto, and the 2-position of thesquarylium ring bearing an O⁻, amino or substituted amino, orsulfonamido group. The mixture is irradiated with infra-red radiationhaving a wavelength within the range of about 700 to about 1200 nm,thereby causing absorption of the radiation by the squarylium dye andformation of acid in the mixture.

BACKGROUND OF THE INVENTION

This invention relates to a process for generation of acid and forimaging, and to an imaging medium for use in this imaging process.

Some conventional non-silver halide photosensitive compositions, forexample photoresists, contain molecules which are inherentlyphotosensitive, so that absorption of a single quantum brings aboutdecomposition of only the single molecule which absorbs the quantum.However, a dramatic increase in the sensitivity of such photosensitivecompositions can be achieved if the photosensitive molecule initiates asecondary reaction which is not radiation-dependent and which effectsconversion of a plurality of molecules for each quantum absorbed. Forexample, photoresist systems are known in which the primaryphotochemical reaction produces an acid, and this acid is employed toeliminate acid-labile groups in a secondary, radiation-independentreaction. See, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706;Reichmanis et al., Chemical Amplification Mechanism forMicrolithography, Chem. Mater., 3(3), 394 (1991) and Berry et al.,Chemically Amplified Resists for I-line and G-line Applications, SPIE,1262, 575 (1990). Also, U.S. Pat. No. 5,084,371 describes aradiation-sensitive mixture which contains a water-insoluble binderwhich comprises a mixture of phenolic and novolak polymers and which issoluble or dispersible in aqueous alkali, and an organic compound whosesolubility in alkaline developer is increased by acid, and which alsocontains at least one acid-cleavable group, and in addition a furthergroup which produces a strong acid upon exposure to radiation.

U.S. Pat. No. 4,916,046 describes a positive radiation-sensitive mixtureusing a monomeric silylenol ether, and a recording medium producedtherefrom. This patent also contains an extensive discussion ofradiation-sensitive compositions which form or eliminate an acid onirradiation. According to this patent, such radiation-sensitivecompositions include diazonium, phosphonium, sulfonium and iodoniumsalts, generally employed in the form of their organic solvent-solublesalts, usually as deposition products with complex acids such astetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonicacid and hexafluoroarsenic acid; halogen compounds, in particulartriazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyroneswhich contain trichloromethyl or tribromomethyl groups; aromaticcompounds which contain ring-bound halogen, preferably bromine; acombination of a thiazole with 2-benzoylmethylenenaphthol; a mixture ofa trihalomethyl compound with N-phenylacridone; α-halocarboxamides; andtribromomethyl phenyl sulfones.

In a reaction of the type in which a primary photochemical reactionproduces and acid and this acid is employed to eliminate acid-labilegroups in a secondary, radiation-independent reaction, if theelimination of the acid-labile groups results in the production of asecond acid (hereinafter called a "secondary" acid), the compoundcontaining such acid-labile groups may hereinafter be referred to as a"secondary acid generator."

A secondary acid generator needs to fulfil several differingrequirements. It is desirable that the material generate a strong acid,since generation of a weak acid, such as the carboxylic acids generatedby some prior art processes, may limit the types of acid-sensitivecompound which can be used. The secondary acid generator is desirably oflow molecular weight in order to reduce the amount of material requiredto generate a specific amount of acid. Finally, the secondary acidgenerator must be compatible with all the other components of theimaging medium in which it is to be used, and should not poseenvironmental problems, such as offensive smell or severe toxicity.

The aforementioned copending application Ser. No. 07/965,172 disclosesthat certain squaric acid derivatives are effective thermal acidgenerators which, upon heating, liberate squaric acid or an acidderivative thereof; thus, these squaric acid derivatives can be used inthermochemical processes for the generation of acid, and for imaging.

It has now been found that breakdown of the squaric acid derivativesdescribed in the aforementioned copending application Ser. No.07/965,172, and of other secondary acid generators, can be catalyzed byacids which can protonate these derivatives, the efficiency of suchprotonation being dependent on the strength of the acid and thusgreatest for very strong acids (superacids); the catalyzed breakdown ofthe secondary acid-generators by superacids occurs rapidly attemperatures significantly lower than those required for uncatalyzedthermal breakdown of the secondary acid generators. Since superacidprecursors are known which generate superacids on exposure to actinic(usually ultra-violet) radiation, a combination of a superacid precursorand one of the aforementioned secondary acid generators allowsradiation-induced generation of relatively strong acid. Thus, thiscombination is useful for generation of acid and for imaging.

SUMMARY OF THE INVENTION

Accordingly, this invention provides an imaging process, which processcomprises:

imagewise exposing a superacid precursor to actinic radiation effectiveto generate superacid from the superacid precursor;

heating the superacid while the superacid is admixed with a secondaryacid generator capable of undergoing thermal decomposition to produce asecondary acid, the thermal decomposition of the secondary acidgenerator being catalyzed by the superacid, the heating being continuedfor a temperature and time sufficient to cause the superacid to producesecondary acid from the secondary acid generator; and

simultaneously with or subsequent to the heating, contacting thesecondary acid with an acid-sensitive material which changes color inthe presence of the secondary acid.

In another aspect, this invention provides a process for generation ofacid, which process comprises:

exposing a superacid precursor to actinic radiation effective togenerate superacid from the superacid precursor; and

heating the superacid while the superacid is admixed with an oxalic acidderivative capable of undergoing thermal decomposition to produce oxalicacid or an acidic derivative thereof, the thermal decomposition of theoxalic acid derivative being catalyzed by the superacid, the heatingbeing continued for a temperature and time sufficient to cause thesuperacid to produce, from the oxalic acid derivative, oxalic acid or anacidic derivative thereof.

This invention also provides an imaging medium comprising:

a superacid precursor capable of generating a superacid upon exposure toactinic radiation; and

a secondary acid generator capable of undergoing thermal decompositionto produce a secondary acid, the thermal decomposition of the secondaryacid generator being catalyzed by the superacid,

the secondary acid generator being selected from the group consistingof:

(a) a 3,4-disubstituted-cyclobut-3-ene-1,2-dione in which at least oneof the 3- and 4-substituents consists of an oxygen atom bonded to thesquaric acid ring, and an alkyl or alkylene group, a partiallyhydrogenated aryl or arylene group, or an aralkyl group, bonded to saidoxygen atom, said 3,4-disubstituted-cyclobut-3-ene-1,2-dione beingcapable of decomposing so as to cause replacement of the or eachoriginal alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkyloxy group ofthe derivative with a hydroxyl group, thereby producing squaric acid oran acidic squaric acid derivative having one hydroxyl group; and

(b) an oxalic acid derivative capable of thermal decomposition to formoxalic acid or an acidic derivative thereof.

Finally, this invention provides an imaging medium comprising:

a superacid precursor capable of generating a superacid upon exposure toactinic radiation;

a secondary acid generator capable of undergoing thermal decompositionto produce a secondary acid, the thermal decomposition of the secondaryacid generator being catalyzed by the superacid; and

an acid-sensitive material which changes color in the presence of thesecondary acid liberated when the secondary acid generator isdecomposed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the accompanying drawings shows a synthesis of a squaric acidderivative of Formula I below; and

FIG. 2 is a schematic cross-section through an imaging medium of thepresent invention as it is being passed between a pair of hot rollersduring the imaging process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As already mentioned, in the present processes a superacid precursor isexposed to actinic (typically ultra-violet) radiation, therebygenerating a superacid from the precursor. The resultant superacid isheated while in admixture with a secondary acid generator capable ofundergoing thermal decomposition to produce a secondary acid. Thisheating is continued for a temperature and time sufficient to cause thesuperacid to catalyze the breakdown of the secondary acid generator, sothat the final quantity of secondary acid present is substantiallylarger than the quantity of superacid produced directly by the actinicradiation acting on the superacid precursor, although of course thesecond acid is typically a weaker acid than the superacid itself. This"chemical amplification" of the superacid by the secondary acidgenerator increases the number of moles of acid generated per einsteinof radiation absorbed, and thus increases the contrast of the imageproduced by the present processes as compared with simple generation ofacid by a superacid precursor.

The term "superacid" is used herein in its conventional sense, that isto say an acid with a pK_(a) less than about 0. Any of the knownsuperacid precursors, for example diazonium, phosphonium, sulfonium andiodonium compounds, may be used in this invention, but iodoniumcompounds are preferred. Especially preferred superacid precursors arediphenyliodonium salts, specifically (4-octyloxyphenyl)phenyliodoniumhexafluorophosphate and hexafluoroantimonate andbis(N-dodecylphenyl)iodonium hexafluoroantimonate.

In the present invention, it is unbuffered superacid which catalyzes thethermal decomposition of the secondary acid generator. It is highlydesirable that the processes of the invention be conducted underessentially anhydrous conditions; as chemists are well aware, the mostpowerful acidic species which can exist in the presence of more than oneequivalent of water is the hydroxonium (hydronium) ion, [H₃ O]⁺.Accordingly, if the medium in which the present process is conductedcontains water, at least part of the superacid produced by the presentprocess will simply generate hydroxonium ion. However, in the absence ofwater, the superacid yields an acidic species much stronger thanhydroxonium ion, and this acidic species effects the acid-catalyzeddecomposition of the secondary acid generator. Typically, the presentprocess is carried out with the superacid precursor and the secondaryacid generator dispersed in a polymeric binder, and such binders canreadily be chosen to provide an essentially anhydrous environment forthe process.

One preferred group of secondary acid generators for use in the presentinvention are 3,4-disubstituted-cyclobut-3-ene-1,2-diones (hereinafterfor convenience referred to as "squaric acid derivatives") in which atleast one of the 3- and 4-substituents consists of an oxygen atom bondedto the squaric acid ring, and an alkyl or alkylene group, a partiallyhydrogenated aryl or arylene group, or an aralkyl group bonded to thisoxygen atom, the 3,4-disubstituted-cyclobut-3-ene-1,2-dione beingcapable of decomposing so as to cause replacement of the or eachoriginal alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group ofthe derivative with a hydroxyl group, thereby producing squaric acid oran acidic squaric acid derivative having one hydroxyl group. Squaricacid and its acidic derivatives are strong acids well suited toeffecting color changes or other effects (for example, polymerization ordepolymerization-reactions) in acid-sensitive materials.

The exact mechanism by which squaric acid or an acidic derivativethereof is formed from a squaric acid derivative by superacid catalyzedthermal decomposition in the present processes may vary depending uponthe type of squaric acid derivative employed. In some cases, for exampledi-t-butyl squarate, one or both groups attached via oxygen atoms to thesquaric acid ring may decompose to yield an alkene or arene, therebyconverting an alkoxy or aryloxy group to a hydroxyl group and formingthe squaric acid or acidic derivative thereof. In other cases, forexample 3-amino-4-(p-vinylbenzyloxy)cyclobut-3-ene-1,2-dione, there isno obvious mechanism for formation of a corresponding alkene or arene,and it appears that the mechanism of acid formation is migration of thevinylbenzyl carbocation or similar group to a different position withinthe molecule (probably to the amino group), and protonation of theremaining oxygen atom to form a hydroxyl group at the position fromwhich the group migrates. In other cases, neither of these pathways ispossible. However, in all cases the net effect is the replacement of thealkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group present inthe original derivative with a hydroxyl group to form squaric acid or anacidic derivative thereof.

Those skilled in the art of organic chemistry will appreciate that thesusceptibility to thermal decomposition of the squaric acid derivativespreferred for use in the present process is related to the stability ofthe cation which is produced from the ester grouping during thedecomposition process. Although the stability of specific cations may beinfluenced by a variety of factors, including steric factors, which maybe peculiar to a particular ester, in general it may be stated that thesquaric acid esters preferred for use in the present process are:

(a) primary and secondary esters of squaric acid in which the α-carbonatom (i.e, the carbon atom bonded directly to the --O-- atom of thesquarate ring) bears a non-basic cation-stabilizing group. Thiscation-stabilizing group may be, for example, an sp² or sp hybridizedcarbon atom, or an oxygen atom;

(b) tertiary esters of squaric acid in which the a-carbon atom does nothave an sp² or sp hybridized carbon atom directly bonded thereto; and

(c) tertiary esters of squaric acid in which the a-carbon atom does havean sp² or sp hybridized carbon atom directly bonded thereto, providedthat this sp² or sp hybridized carbon atom (or at least one of these sp²or sp hybridized carbon atoms, if more than one such atom is bondeddirectly to the α-carbon atom) is conjugated with anelectron-withdrawing group.

It will be apparent to skilled organic chemists that, provided one ofthe aforementioned types of ester groupings is present in the squaricacid derivative to produce one hydroxyl group after thermaldecomposition, the group present in place of the other hydroxyl group ofsquaric acid is of little consequence, provided that this other groupdoes not interfere with the thermal decomposition. Indeed, the widevariation possible in this other group has the advantage that this groupcan be varied to control other properties of the derivative, for exampleits compatibility with other components of the imaging medium, or itssolubility in solvents used to form coating solutions used in thepreparation of the imaging medium.

Examples of squaric acid derivatives useful in the present processesinclude:

(a) those of the formula: ##STR1## in which R¹ is an alkyl group, apartially hydrogenated aromatic group, or an aralkyl group, and R² is ahydrogen atom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino,alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino,dialkylphosphoxy or trialkylsilyl group, subject to the proviso thateither or both of the groups R¹ and R² may be attached to a polymer.Among the derivatives of Formula I, especially preferred groups arethose in which (a) R¹ is an unsubstituted or phenyl substituted alkylgroup containing a total of not more than about 20 carbon atoms, and R²is an alkyl group containing not more than about 20 carbon atoms, or aphenyl group (which may be substituted or unsubstituted); and (b) R¹ isa benzyl group and R² is an amino group.

(b) those of the formula: ##STR2## in which R¹ and R³ independently areeach an alkyl group, a partially hydrogenated aryl group or an aralkylgroup, subject to the proviso that either or both of the groups R¹ andR³ may be attached to a polymer. Among the derivatives of Formula II, anespecially preferred group are those in which R¹ and R³ are eachindependently an unsubstituted or phenyl substituted alkyl groupcontaining a total of not more than about 20 carbon atoms. Specificpreferred compounds of Formula II are those in which R¹ and R³ are eacha tertiary butyl group, a benzyl group, an α-methylbenzyl group or acyclohexyl group, namely di-tertiary butyl squarate, dibenzyl squarate,bis(α-methylbenzyl) squarate and dicyclohexyl squarate.

(c) those of the formula: ##STR3## in which n is 0 or 1, and R⁴ is analkylene group or a partially hydrogenated arylene group. Among thederivatives of Formula III, an especially preferred group are those inwhich n is 1 and R⁴ is an alkylene group containing not more than about12 carbon atoms.

(d) those having at least one unit of the formula: ##STR4## in which nis 0 or 1, and R⁵ is an alkylene or partially hydrogenated arylenegroup. In addition to the fragmentable groups R⁵, the compounds may alsocontain one or more units in which a non-fragmentable group is attachedto a squarate ring, directly or via an oxygen atom.

The squaric acid derivatives of Formula IV include not only highpolymers, but also dimers, trimers, tetramers, etc. including at leastone of the specified units. The terminating groups on the derivatives ofFormula IV may be any of groups OR¹ or R² discussed above with referenceto Formula I. Thus, for example, Formula IV includes the squaric aciddimer derivative of the formula: ##STR5##

The squaric acid derivatives of Formulae I and II are usually monomeric.However, these derivatives of Formulae I and II can be incorporated intopolymers by having at least one of the groups R¹, R² and R³ attached toa polymer. Attachment of the squaric acid derivatives to a polymer inthis manner may be advantageous in that it may avoid incompatibilityand/or phase separation which might occur between a monomeric squaricacid derivative of Formula I or II and a polymeric binder needed in animaging medium.

The attachment of the groups R¹, R² and R³ to a polymer may be effectedin various ways, which will be familiar to those skilled in the art ofpolymer synthesis. The squaric acid derivatives may be incorporated intothe backbone of a polymer, for example in a polymer similar to the dimerof the formula given above. Alternatively, the squaric acid derivativesmay be present as sidechains on a polymer; for example, one of thegroups R¹, R² and R³ could contain an amino group able to react with apolymer containing a carboxyl groups or derivatives thereof to form anamide linkage which would link the squaric acid derivative as asidechain on to the polymer, or these groups may contain unsaturatedlinkages which enable the squaric acid derivatives to be polymerized,either alone or in admixture with other unsaturated monomers.

In the present process, it is generally undesirable to form substantialquantities of gas during the superacid-catalyzed decomposition of thesquaric acid derivative (or other secondary acid generator), since suchgas may distort the medium containing the squaric acid derivative orform vesicles therein, and such distortion or vesicle formation mayinterfere with proper image formation. Accordingly, if the decompositionof the squaric acid derivative yields an alkene, it is desirable thatthe groups R¹, R³, R⁴ and R⁵ be chosen so that this alkene is a liquidat 20° C., and preferably higher, since some heating of the alkene willinevitably occur during the superacid-catalyzed decomposition. In somecases, however, the alkene liberated may be sufficiently soluble in themedium containing the squaric acid derivative that liberation of ahighly volatile alkene will not result in distortion of, or vesicleformation in, the medium.

Another preferred group of secondary acid generators for use in thepresent process are oxalic acid derivatives which undergosuperacid-catalyzed breakdown to give oxalic acid or an acidicderivative thereof, for example an oxalic acid hemiester. Althoughoxalic acid and its acidic derivatives are not quite such strong acidsas squaric acid and its acidic derivatives, oxalic acid and itsderivatives are sufficiently strong acids for most purposes for whichsecondary acids are required in the present process. Also, oxalic acidderivatives are, in general, less costly than squaric acid derivatives.

The types of oxalic acid derivatives preferred for use in the presentprocess are rather more diverse in structure than the squaric acidderivatives, and the choice of oxalic acid derivative for any specificprocess may be governed more by the thermal breakdown properties of thederivative than its exact chemical structure; in general, for practicalreasons such as the limited temperature range to which other componentsof the imaging medium-may safely be exposed, it is preferred that theoxalic acid derivative be one which begins to decompose thermally at atemperature in the range of about 140° to about 180° C., as measured bydifferential scanning calorimetry in a nitrogen atmosphere at a 10°C./minute temperature ramp, in the absence of any catalyst. Since thepresence of a superacid catalyst lowers the thermal decompositiontemperature of oxalic acid derivatives by at least about 20° C. andpotentially significantly more, derivatives which decompose uncatalyzedat about 140° to about 180° C., will, in the presence of superacid,decompose at temperatures as low as about 65° C., temperatures to whichother components of the imaging medium can in general be exposed.

The factors affecting the ability of the oxalic acid derivatives toundergo superacid-catalyzed thermal decomposition are similar to thoseaffecting the ability of the aforementioned squaric acid derivatives toundergo the same reaction, and thus the preferred ester groups are ofthe same types. Accordingly, preferred oxalic acid derivatives for usein the present process include:

(a) primary and secondary esters of oxalic acid in which the α-carbonatom (i.e, the carbon atom bonded directly to the --O-- atom of theoxalate grouping) bears a non-basic cation-stabilizing group. Thiscation-stabilizing group may be, for example, an sp² or sp hybridizedcarbon atom, or an oxygen atom;

(b) tertiary esters of oxalic acid in which the α-carbon atom does nothave an sp² or sp hybridized carbon atom directly bonded thereto; and

(c) tertiary esters of oxalic acid in which the α-carbon atom does havean sp² or sp hybridized carbon atom directly bonded thereto, providedthat this sp² or sp hybridized carbon atom (or at least one of these sp²or sp hybridized carbon atoms, if more than one such atom is bondeddirectly to the α-carbon atom) is conjugated with anelectron-withdrawing group.

(d) an ester formed by condensation of two moles of an alcohol with thebis(hemioxalate) of a diol, provided that the ester contains at leastone ester grouping of types (a), (b) or (c) above. One example of anester of this type is that of the structure: ##STR6## which can beregarded as formed from two moles of menthol(2-methylethyl-4-methylcyclohexanol) and one mole of thebis(hemioxalate) of 1,6-bis-(4-hydroxymethylphenoxy)hexane. Since thestructure of the central residue of the diol in such esters can varywidely, the solubility and other properties of the esters can be "tuned"as required for compatibility with other components of the imagingmedium, while the nature of the end groups, which undergo theacid-forming thermal decomposition, can be varied independently of thenature of the central residue.

(e) polymeric oxalates derived from polymerization of oxalate estershaving an ethylenically unsaturated group, provided that the estercontains at least one ester grouping of types (a), (b) or (c) above. Aswith the squaric acid derivatives discussed above, use of a polymericoxalate rather than a monomeric one may be advantageous in that it mayavoid incompatibility and/or phase separation which might occur betweena monomeric derivative and a polymeric binder needed in an imagingmedium. Use of a polymeric derivative also tends to inhibit diffusion ofthe oxalate through the imaging medium during storage prior to imaging.Although polymeric oxalates can be formed in other ways, at present weprefer to form such oxalates by first forming an oxalate ester in whichone of the ester groupings comprises an ethylenically unsaturated group,and then polymerizing this ester using a conventional free radicalpolymerization initiator, for example azobis(isobutyronitrile) (AIBN).The ethylenically unsaturated group is conveniently an acrylate ormethacrylate group, while the other ester grouping in the monomericoxalate can be any of the types discussed above.

(f) Condensation polymers of oxalates, provided that the ester containsat least one ester grouping of types (a), (b) or (c) above. This type ofpolymer also possesses the advantages discussed under (e) above.

Although the present process may be used for other purposes, such astriggering an acid-catalyzed chemical reaction (for example,polymerization or depolymerization reactions), it is primarily intendedfor use in image formation processes, and thus, simultaneously with orsubsequent to the heating of the secondary acid generator and thesuperacid, the secondary acid liberated is desirably contacted with anacid-sensitive material which changes color in the presence of thesecondary acid. (It will be appreciated that the "color change" involvedin such an imaging process need not be a visible color change. If, forexample, the present process is used to provide security markingsintended to be machine-readable, the "color change" could be a change inabsorption from one non-visible wavelength to another, such that it canbe detected by the appropriate machine-reading device.) Also, desirablythe exposure of the superacid precursor to the actinic radiation iseffected in an imagewise manner so that the color change of theacid-sensitive material occurs only in areas which have been exposed toactinic radiation, thereby forming an image.

The acid-sensitive material used in the process of the present inventionmay be any material which undergoes a color change in the presence ofacid. Thus any conventional indicator dye may be used as theacid-sensitive material, as may the leuco dyes disclosed in theaforementioned U.S. Pat. Nos. 4,602,263; 4,720,449 and 4,826,976, whichare also sensitive to acid.

To prevent premature color formation in an imaging process of thepresent invention prior to the exposure step, and thus avoid theincrease in D_(min) which may occur when some prior art imaging mediaare stored for long periods before use, advantageously, prior to theexposure step, the acid-sensitive material is in admixture with anamount of a basic material insufficient to neutralize all the acidcapable of being liberated by the superacid precursor. The provision ofthis basic material serves to "soak up" minor amounts of acid which maybe generated by decomposition of the superacid precursor by, forexample, accidental exposure of the medium to ultraviolet light duringtransportation and storage. Upon exposure, the large amount of acidgenerated by the superacid precursor and secondary acid generatoroverwhelms the amount of basic material, leaving excess acid whicheffects the color change in the acid-sensitive material.

The exposure of the superacid precursor to the actinic (typicallyultraviolet) radiation can be effected in any of the ways conventionallyused for exposing media to the same type of radiation. Thus, forexample, the present medium can be exposed using the G-line from amercury arc lamp. In some cases, it may be convenient to employ anultra-violet laser. The use of a laser is a convenient way to recorddata as an image pattern in response to transmitted signals, such asdigitized information.

Some imaging media of the present invention (for example those intendedfor use as photoresists and containing polymerizable monomers oroligomers or depolymerizable polymers) may comprise only a single layercontaining all the components of the imaging medium. However, mediacontaining an acid-sensitive material desirably comprise two separatelayers or phases, so that, prior to the heating, the acid-sensitivematerial is present in a layer or phase separate from the layer or phasecontaining the superacid precursor and the secondary acid generator, andfollowing the generation of the secondary acid from the secondary acidgenerator, the two layers or phases are mixed, thereby effecting thecolor or other change in the acid-sensitive material.

In principle, the mixing of the acid-sensitive material with thesuperacid precursor and secondary acid generator should be effectedafter the generation of the secondary acid from the secondary acidgenerator. However, in practice if the superacid precursor and secondaryacid generator are present in one layer of a two-layer imaging medium,and the acid-sensitive material in the other layer of the medium, thesetwo layers being such that their diffusible components mix on heating,both the generation of the secondary acid and the mixing of the twolayers may be effected in a single heating step, since thesuperacid-catalyzed decomposition of the secondary acid generator willtypically be essentially complete before mixing of the two layersbecomes significant.

When a two-layer structure is used, it is not necessary that the twolayers be affixed to one another before imaging. The production ofsecondary acid in exposed regions effected by the present process is a"permanent" chemical change, and hence it is possible to delaycontacting the exposed medium with an acid-sensitive material for asubstantial time. (Obviously, excessive delay may reduce the quality ofan image produced by allowing secondary acid to diffuse from exposedinto unexposed areas of the medium.) Accordingly, the two layers of theimaging medium may be laminated together after the second irradiation.However, in general it is most convenient to form the two layers bycoating one on the other, or laminating the two layers together beforeimaging, since in this way only a single sheet of material has tohandled during the imaging process. Since it is important that the twolayers not mix prematurely, if the two layers are to be coatedsuccessively on to a support, it is usually desirable to coat one layerfrom an aqueous medium and the other from a non-aqueous medium.Typically, the layer containing the superacid precursor is coated froman organic solution and the layer containing an acid-sensitive leuco dyeor other material is coated from an aqueous dispersion.

As already mentioned, prior to the heating step, the acid-sensitivematerial may be in admixture with an amount of a basic materialinsufficient to neutralize all the secondary acid liberated by thesecondary acid generator during the heating, so that the secondary acidliberated by the secondary acid generator during the heating neutralizesall of the basic material and leaves excess secondary acid sufficient toeffect the change in the acid-sensitive material. The provision of thisbasic material serves to "soak up" minor amounts of acid which may begenerated in unexposed areas after exposure due, for example, to slowdecomposition of the superacid precursor during protracted storage.Since obviously the basic material cannot be allowed to contact thesuperacid prior to the heating step, desirably the acid-sensitivematerial is present in a layer or phase separate from the layer or phasecontaining the superacid precursor and the secondary acid generator and,following the generation of the secondary acid, the two layers or phasesare mixed, thereby effecting the change in the acid-sensitive material.

In addition to the two aforementioned layers containing the superacidprecursor, secondary acid generator and acid-sensitive material, theimaging media of the present invention may comprise a support andadditional layers, for example, a subbing layer to improve adhesion tothe support, interlayers for thermally insulating multiple imaginglayers from one another, an anti-abrasive topcoat layer, and otherauxiliary layers.

The support employed may be transparent or opaque and may be anymaterial that retains its dimensional stability at the temperature usedduring the heating step. Suitable supports include paper, paper coatedwith a resin or pigment, such as, calcium carbonate or calcined clay,synthetic papers or plastic films, such as polyethylene, polypropylene,polycarbonate, cellulose acetate and polystyrene. The preferred materialfor the support is a polyester, desirably poly(ethylene terephthalate).

Usually the layer or layers containing the superacid precursor,secondary acid generator and acid-sensitive material will also contain abinder; typically this layer or these layers are formed by combining theactive materials and the binder in a common solvent, applying a layer ofthe coating composition to the support and then drying. Rather than asolution coating, the layer may be applied as a dispersion or anemulsion. The coating composition also may contain dispersing agents,plasticizers, defoaming agents, coating aids and materials such as waxesto prevent sticking.

The binder used for the layer(s) in which superacid is to be generatedmust of course be non-basic, such that the superacid is not buffered bythe binder. Examples of binders that may be used include methylcellulose, cellulose acetate butyrate, styrene-acrylonitrile copolymers,polystyrene, poly(α-methylstyrene), copolymers of styrene and butadiene,poly(methyl methacrylate), copolymers of methyl and ethyl acrylate,poly(vinyl acetate), poly(vinyl butyral), polycarbonate poly(vinylidenechloride) and poly(vinyl chloride). It will be appreciated that thebinder selected should not have any adverse effect on the superacidprecursor, secondary acid generator or the acid-sensitive materialincorporated therein. Also, the binder should be heat-stable at thetemperatures encountered during the heating step and should betransparent so that it does not interfere with viewing of the image. Thebinder must of course transmit the actinic radiation used to expose themedium.

In forming the layer containing the secondary acid generator,temperatures should be maintained below levels that will initiatethermal decomposition of the secondary acid generator so that theacid-sensitive material will not be prematurely colored or bleached.

The squaric acid derivatives preferred for use in the processes of thepresent invention can be prepared by known methods, such as thosedescribed in U.S. Pat. No. 4,092,146 and Tetrahedron Letters (1977),4437-38, and 23, 361-4, and Chem. Ber. 121, 569-71 (1988) and 113, 1-8(1980). In general, the diesters of Formula II can be prepared byreacting disilver squarate with the appropriate alkyl halide(s),preferably the alkyl bromides. The ester groupings may be varied byroutine transesterification reactions, or by reacting the diacidchloride of squaric acid with an appropriate alcohol or alkoxide:

The squaric acid derivatives of Formula I in which R² is an alkyl,cycloalkyl, aralkyl or aryl group can be prepared from derivatives ofFormula II by the synthesis shown in FIG. 1. The diester of Formula IIis first condensed with a compound containing a negatively chargedspecies R² ; this compound is normally an organometallic compound, andpreferably an organolithium compound. The reaction adds the --R² groupto one of the oxo groups of the diester to produce the squaric acidderivative of Formula VI; to avoid disubstitution into both oxo groups,not more than the stoichiometric amount of the organometallic reagentshould be used.

After being separated from unreacted starting material and otherby-products, the squaric acid derivative VI is treated with an acid, forexample hydrochloric acid, to convert it to the desired squaric acidderivative I. Although it is possible to simply add acid to the reactionmixture resulting from the treatment of the diester with theorganometallic reagent, this course is not recommended, since thesquaric acid derivative I produced may be contaminated with unreacteddiester, and the diester and squaric acid derivative I are so similarthat it is extremely difficult to separate them, even by chromatography.

It will be appreciated that the synthesis shown in FIG. 1 may bemodified in various ways. If, for example, the nature of the group R¹desired in the final compound of Formula I is such that it would reactwith the organometallic reagent, the reactions shown in FIG. 1 may becarried out with a diester in which the ester groupings do not containthe group R¹, and the final product of Formula I may be subjected totransesterification or other reactions to introduce the group R¹.

The derivatives of Formula I in which R² is an amino, alkylamino ordialkylamino group can be prepared by similar methods from squaric aciddiesters. For example, as illustrated in the Examples below, reaction ofbis(4-vinylbenzyl) squarate with methylamine gives3-amino-4-(p-vinylbenzyloxy)cyclobut-3-ene-1,2-dione. Analogous methodsfor the synthesis of the other compounds of Formula I will readily beapparent to those skilled in the art of organic synthesis.

The forms of the squaric acid derivative of Formulae I and II in whichat least one of R¹, R² and R³ is attached to a polymer may be preparedby reactions analogous to those used to prepare the monomericderivatives of Formulae I and II, for example by treating a polymercontaining appropriate alkoxide groups with the diacid chloride or amonoester monoacid chloride of squaric acid. Alternatively, thesepolymer-attached derivatives may be prepared by transesterification, forexample by treating a polymer containing esterified hydroxyl groups witha monomeric squaric acid derivative of Formula I or II. Other methodsfor attachment of these derivatives to polymers, or inclusion of thesederivatives into polymer backbones, have already been discussed above.

The derivatives of Formula III may be prepared by transesterificationfrom derivatives of Formula II, or another squaric acid diester, and theappropriate diol.

The monomeric oxalic acid derivatives useful in the present process canbe prepared by routine esterification techniques which will be familiarto those skilled in organic synthesis, and several Examples of suchtechniques are exemplified in detail below. The preparation of polymericoxalic acid derivatives has already been discussed.

A preferred embodiment of the invention will now be described, though byway of illustration only, with reference to FIG. 2 of the accompanyingdrawings, which shows a schematic cross-section through an imagingmedium (generally designated 10) of the invention as the image thereinis being fixed by being passed between a pair of hot rollers 12.

The imaging medium 10 comprises a support 14 formed from a plastic film.Typically the support 14 will comprise a polyethylene terephthalate film3 to 10 mils (76 to 254 mμ) in thickness, and its upper surface (in FIG.2) may be treated with a sub-coat, such as is well-known to thoseskilled in the preparation of imaging media, to improve adhesion of theother layers to the support.

On the support 14 is disposed an acid-forming layer 16 comprising asuperacid precursor and a secondary acid generator from which asecondary acid can be liberated by a superacid. The acid-forming layer16 may also contain a small amount of a basic material to neutralize anyacid produced by breakdown of the superacid precursor or secondary acidgenerator prior to exposure of the medium. On the opposed side of theacid-forming layer 16 from the support 14 is disposed an imaging layer18 comprising an acid-sensitive material, which changes color in thepresence of secondary acid, and a small amount of a base, which servesto neutralize any acid produced by breakdown of the superacid precursorin the acid-forming layer 16. The acid-forming layer 16 and the imaginglayer 18 both contain a binder having a glass transition temperaturesubstantially above room temperature.

Finally, the imaging medium comprises an abrasion-resistant topcoat 20.

The imaging medium 10 may be formed by coating the layers 16, 18 and 20on to the support 14. Alternatively, for example, the layers 16 and 18may be coated on to the support 14, and the topcoat 20 laminated on tothe resultant structure.

The imaging medium 10 is exposed by writing on selected areas of themedium with an ultra-violet laser or using incoherent ultra-violetradiation with exposure being effected through a mask; the exposure ofthe medium 10 may be effected through the topcoat 20, as indicated bythe arrow 22 in the drawing. Within the exposed regions of theacid-forming layer 16, the exposure to ultra-violet radiation causesbreakdown of the superacid precursor with the formation of thecorresponding superacid. After exposure, the imaging medium 10 is passedbetween the heated rollers 12. The heat and pressure applied by therollers 12 causes the superacid present in the exposed regions of theacid-forming layer 16 to bring about catalytic breakdown of thesecondary acid generator therein, thereby causing formation of aquantity of secondary acid substantially larger than the quantity ofsuperacid originally generated by the ultra-violet radiation. The heatand pressure applied by the rollers 12 also heats the color-forminglayer 18 and the acid-forming layer 16 above their glass transitiontemperatures, thereby causing the diffusible components of these twolayers to become intermixed so that, in exposed regions, the acidproduced in the acid-forming layer 16 effects the color change of theacid-sensitive material, thereby forming an image.

The following Examples are now given, though by way of illustrationonly, to show details of preferred reagents, conditions and techniquesused in the process and imaging medium of the present invention.

Examples 1-11:

Preparation of squaric acid derivative secondary acid generators

3,4-Bis(t-butoxy)cyclobut-3-ene-1,2-dione ("bis t-butyl squarate";hereinafter referred to as "Compound A") used in certain Examples belowwas prepared as described in E. V. Dehmlow et al., Chem. Ber. 113, 1-8(1980). 3,4-Bis(benzyloxy)cyclobut-3-ene-1,2-dione ("dibenzyl squarate";hereinafter referred to as "Compound B") used in certain Examples belowwas prepared as described in N. Islam et al, Tetrahedron 43, 959-970(1987). Silver squarate was prepared as described in S. Cohen et al., J.Am. Chem. Soc., 88, 5433 (1966).

Example 1

Preparation of bis(3-bromo-2,3-dimethylbut-2-yl) squarate

This Example illustrates the preparation of3,4-bis(3-bromo-2,3-dimethylbut-2-oxy)-cyclobuto-3-ene-1,2-dione("bis(3-bromo-2,3-dimethylbut-2-yl) squarate"), the compound of FormulaII in which R¹ and R³ are each a 3-bromo-2,3-dimethylbut-2-yl group.

Silver squarate (1.0 g, 3.0 mmole) was added to a solution of2,3-dibromo-2,3-dimethylbutane (1.0 g, 4.0 mmole) in dry ether (3 mL) atroom temperature. The suspension became warm, and was cooled by a waterbath at room temperature. After six hours stirring, the precipitateremaining was removed by filtration, and washed with ether. The combinedether extracts were concentrated, and the crude product obtainedtherefrom was purified by flash chromatography on silica gel with 1:3ether/hexanes as eluent to give the diester (140 mg, 11% yield) as awhite powder which decomposed at 131°-132° C. The structure of thecompound was confirmed by mass spectroscopy and by ¹ H and ¹³ NMRspectroscopy.

Example 2

Preparation of 3-t-butoxy-4-phenylcyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3-t-butoxy-4-phenylcyclobut-3-ene-1,2-dione, the compound of Formula Iin which R¹ is a tertiary butyl group and R² is a phenyl group.

Phenyl magnesium bromide (4.6 mL of a 1.0M solution in THF, 4.6 mmole)was added dropwise over a period of 5 minutes to a solution ofdi-t-butyl squarate (1.0 g, 4.42 mmole) in dry ether (10 mL) at -78° C.under nitrogen. After 30 minutes, the reaction mixture was warmed to 0°C., and stirred at this temperature for an additional one hour. Water(10 mL) and ether (10 mL) were then added to the reaction mixture andthe layers were separated. The aqueous layer was extracted twice withdichloromethane. The combined organic layers were dried over magnesiumsulfate and concentrated, to give a yellow oil (1.43 g), whichcrystallized. The resultant material was dissolved in dichloromethane(25 mL) and concentrated hydrochloric acid (4 drops) was added, withstirring, to this solution at room temperature. After 30 minutes, afurther four drops of concentrated hydrochloric acid were added.Dichloromethane (25 mL) was added, and the resultant solution was washedwith a saturated solution of sodium bicarbonate and then with brine,dried over magnesium sulfate, and concentrated. The crude product thusobtained was purified by flash chromatography on silica gel with tolueneas eluent. The chromatographed material was further purified byrecrystallization from toluene/hexanes to give the desired monoester asyellow crystals (142 mg, 14% yield) which decomposed at 105°-110° C. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 3

Preparation of 3,4-bis(α-methylbenzyloxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3,4-bis(α-methylbenzyloxy)-cyclobut-3-ene-1,2-dione-("bis(α-methylbenzyl)squarate"; hereinafter referred to as "Compound C"), the compound ofFormula II in which R¹ and R³ are each an α-methylbenzyl group.

1-Bromo-1-phenylethane (3.1 g, 16.8 mmole) was added dropwise to asuspension of silver squarate (2.5 g, 7.62 mmole) in dry ether (40 mL)at 0° C. After the addition was complete, the reaction mixture wasallowed to warm to room temperature and was stirred for four hours inthe dark. The solid remaining after this time (silver bromide) wasremoved by filtration and washed with more ether. The combined ethersolutions were washed with a saturated solution of sodium bicarbonateand dried over sodium sulfate. Evaporation of the solvent was followedby purification by flash chromatography on silica gel with 0-60%ether/hexanes as eluant to give the desired diester (394 mg, 16% yield)as a colorless oil. The diester was obtained as a mixture ofdiastereoisomers which were not separable by this type ofchromatography. The structure of the diester was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 4

Preparation of 3,4-bis(p-methylbenzyloxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3,4-bis(p-methylbenzyloxy)-cyclobut-3-ene-1,2-dione("bis(p-methylbenzyl) squarate", hereinafter called "Compound D"), thecompound of Formula II in which R¹ and R³ are each a p-methylbenzylgroup.

Triethylamine (0.93 g, 9.2 mmole) was added to a stirred suspension ofsquaric acid (0.5 g, 4.38 mmole) in chloroform (10 mL) and the resultantsolution was cooled with an ice/water bath. A solution ofct-bromo-p-xylene (2.03 g, 11.0 mmole) in chloroform (10 mL) was thenadded dropwise over a period of 30 minutes. After this time, the coolingbath was removed and the solution was held at room temperature for 4.5hours. The reaction mixture was then diluted with chloroform (20 mL),washed successively with a saturated aqueous solution of sodiumbicarbonate (2×20 mL) and saturated brine (20 mL), dried over magnesiumsulfate and concentrated under reduced pressure. The-resultant oil wasfurther purified by partition between ether (50 mL) and saturatedaqueous sodium bicarbonate (20 mL) and separation of the organic layer.The organic layer was washed successively with a saturated aqueoussolution of sodium bicarbonate (20 mL) and saturated brine (20 mL),dried over magnesium sulfate and concentrated under reduced pressure.The oil which resulted was crystallized from hot hexanes (20 mL) to givethe desired compound (300 mg, 21.3% yield) as off-white crystals. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 5

Preparation of 3,4-bis(cyclohexyloxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3,4-bis(cyclohexyloxy)-cyclobut-3-ene-1,2-dione ("dicyclohexylsquarate", hereinafter called "Compound E"), the compound of Formula IIin which R¹ and R³ are each a cyclohexyl group.

Cyclohexyl bromide (9.95 g, 61 mmole) was added dropwise over a periodof 20 minutes to a stirred suspension of silver squarate (4.0 g, 12.2mmole) in ether (80 mL) in the dark with ice/water cooling. The ice bathwas then removed and the reaction mixture was stirred overnight at roomtemperature, then filtered to remove silver bromide, and the residue waswashed with ether (2×20 mL). The ether solutions were combined andwashed successively with a saturated aqueous solution of sodiumbicarbonate (50 mL) and saturated brine (50 mL), dried over magnesiumsulfate and concentrated under reduced pressure to give the desiredcompound as a viscous oil which solidified upon storage in arefrigerator to give an off-white solid (0.55 g, 16% yield). Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 6

Preparation of 3-amino-4-(t-butoxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3-amino-4-(t-butoxy)-cyclobut-3-ene-1,2-dione (hereinafter called"Compound F"), the compound of Formula I in which R¹ is a tertiary butylgroup and R² is an amino group.

A stream of ammonia gas was passed into a stirred solution of Compound A(0.7 g, 3.07 mmole) in methanol (40 mL) for 2 minutes. The solution wasthen allowed to stand at room temperature for 1 hour, during which timea small amount of insoluble material was precipitated. The sediment wasremoved by filtration, and the solvent was removed under reducedpressure to yield a yellow solid, which was washed with ether (2×50 mL)to remove starting material and butanol (0.16 g of impurities werecollected, after solvent evaporation). The solid which remained wasdissolved in dichloromethane (150 mL) and the solution was filtered.Removal of the solvent under reduced pressure yielded the desiredcompound as white crystals (0.25 g, 48% yield) which melted at 220°-225°C. The structure of this compound was confirmed by ¹ H NMR spectroscopy.

Example 7

Preparation of 4-hexyl-3-(p-vinyl-benzyloxy)cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of4-hexyl-3-(p-vinylbenzyl-oxy)-cyclobut-3-ene-1,2-dione (hereinaftercalled "Compound G"), the compound of Formula I in which R² is a hexylgroup and R¹ is an p-vinylbenzyl group.

Part A: Preparation of 2,3-dibutoxy-4-hexyl-4-hydroxycyclobut-2-en-1-one

Hexyl magnesium bromide (40 mL of a 2M solution in ether, 80.0 mmole)was added dropwise over a period of 45 minutes to a solution ofdi-n-butyl squarate in dry THF (150 mL) at -78° C. under nitrogen, andthe reaction mixture was held at that temperature for 1 hour. Thereaction mixture was then allowed to warm to room temperature arestirred for an additional 3 hours, after which time it was cooled usingan ice/water bath, and quenched by the addition of water (25 mL) addeddropwise over a period of 5 minutes. Saturated brine (300 mL) and ether(300 mL) were then added, the layers were separated, and the aqueouslayer was extracted with additional ether (300 mL). The ether extractswere combined and dried over magnesium sulfate, and the solvents wereremoved to give a golden oil (15.64 g) containing the desired product;this oil was used without further purification in Part B below.

Part B: Preparation of 3-hexyl-4-hydroxy-cyclobut-3-ene-1,2-dione

6N Hydrochloric acid (150 mL) was added in one portion to a stirredsolution of crude 2,3-dibutoxy-4-hexyl-4-hydroxycyclobut-2-en-1-one(15.1 g, prepared in Part A above) in THF (150 mL), and the resultantsolution was stirred at room temperature for 3 hours. The reactionmixture was then concentrated under reduced pressure to give a yellowsolid. To this solid was added water (100 mL), which was then removedunder reduced pressure. Toluene (100 mL) was similarly added and removedunder reduced pressure, and then dichloromethane (200 mL) was added tothe residue and the resultant solution was filtered and concentrated toproduce a yellow oil. Hexanes (200 mL) were added and the resultantsolution was cooled to induce crystallization. After recrystallizationfrom hexanes, the desired compound was isolated as tan crystals (4.28 g,33% yield over Parts A and B). The structure of this compound wasconfirmed by mass spectroscopy and by ₁ H and ¹³ C NMR spectroscopy.

Part C: Preparation of4-hexyl-3-(p-vinylbenzyloxy)-cyclobut-3-ene-1,2-dione

Triethylamine (1.75 g, 17.3 mmole), 2,6-di-t-butyl-4-methylphenol (aradical inhibitor, 0.7 mg, 3.41 μmol) and 4-vinylbenzyl chloride (5.04g, 33 mmole) were added, in that order, to a solution of3-hexyl-4-hydroxy-cyclobut-3-en-1,2-one (3.0 g, 16.5 mmole, prepared inPart B above) in chloroform (90 mL), and the resultant solution washeated at reflux for 7 hours. The solution was then cooled and allowedto stand overnight at room temperature, after which it was heated atreflux for a further 7 hours, then cooled and allowed to stand overnighta second time. The reaction mixture was then concentrated under reducedpressure, the residue dissolved in dichloromethane (150 mL), and theresultant solution washed with water (2×75 mL), dried over magnesiumsulfate and concentrated under reduced pressure to yield a yellow oil,which was purified by short-path distillation (to remove excess4-vinylbenzyl chloride) at 72°-74° C. and 1.7 mm Hg pressure. Theresidue from the distillation was purified by flash chromatography onsilica gel with dichloromethane as eluant to give the desired compound(1.23 g, 25% yield) as a golden oil. The structure of this compound wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 8

Preparation of3-methylamino-4-(p-vinyl-benzyloxy)cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3-methylamino-4-(p-vinylbenzyloxy)-cyclobut-3-ene-1,2-dione (hereinafterreferred to as "Compound H"), the compound of Formula I in which R² isan amino group and R¹ is a p-vinylbenzyl group.

Part A: Preparation of bis(4-vinylbenzyl) squarate

4-Vinylbenzyl chloride (13 g, 85 mmole) was added to a suspension ofsilver squarate (5.5 g, 48 mmole) in dry ether (100 mL), and theresultant mixture was stirred in the dark for 3 days. The reactionmixture was then filtered and the solvent removed under reducedpressure. The residue was taken up in dichloromethane methane andfiltered through a short column of silica gel, then concentrated underreduced pressure, to yield the desired compound in a crude form, whichwas used in Part B below without further purification.

Part B: Preparation of3-methylamino-4-(p-vinylbenzyloxy)-cyclobut-3-ene-1,2-dione

The crude product from Part A above was dissolved in ether (300 mL) andgaseous methylamine was bubbled through this ether solution for 1minute. The resultant mixture was allowed to stand for 5 minutes, thenthe precipitate which had formed was removed by filtration, redissolvedin chloroform and filtered through Celite (manufactured byJohns-Manville Corporation, Denver, Colo. 80217). The solvent wasremoved under reduced pressure to give the desired product (hereinaftercalled "Compound H") as a white solid, melting point 152° C. (3.5 g, 30%yield over Parts A and B). The structure of this compound was confirmedby ¹ H NMR spectroscopy.

Example 9

Preparation of copolymer of Compound H with lauryl methacrylate

This Example illustrates the preparation of a 1:1 w/w copolymer ofCompound H prepared in Example 8 above with lauryl methacrylate.

Compound H (1 g) and lauryl methacrylate (1 g) were dissolved in amixture of 2-propanol (30 mL) and ethanol (20 mL), and the resultantsolution was purged with nitrogen. Azoisobutyronitrile (0.01 g) was thenadded, and the solution was held at 65° C. overnight, during which timea precipitate (250 mg) formed. This precipitate was collected and shownby infra-red spectroscopy to contain squarate esters.

Example 10

Preparation of4-[5-[1,2-dioxo-3-hydroxycyclobut-3-en-4-yl]pent-1-yl]-3-hydroxycyclobut-3-ene-1,2-dione

Pentamethylenebis(magnesium bromide) (25 mL of a 0.5M solution in THF,12.5 mmole) was added dropwise over a period of 15 minutes to a solutionof dibutyl squarate (5.66 g, 25 mmole) in dry THF (50 mL) at -78° C.under a stream of nitrogen. The resulting suspension was stirred at -78°C. for 1 hour, then allowed to warm to room temperature and stirred fora further 2 hours. The homogeneous yellow solution which resulted wascooled to 0° C., and water (10 mL) was added dropwise over a period of 2minutes. After standing for 5 minutes, the solution was diluted with THF(50 mL) and washed with saturated sodium chloride solution (150 mL). Anemulsion was formed, which was separated by evaporative removal of THFand addition of dichloromethane (200 mL). The organic layer wasseparated and the aqueous layer was extracted with more dichloromethane(100 mL). The combined dichloromethane layers were dried over magnesiumsulfate and concentrated under reduced pressure to yield a golden oilwhich was shown by thin layer chromatography, on silica gel with 1:1ether/hexanes as eluent, to consist of five components.

This mixture was separated by flash chromatography on silica gel with1:1 ether/hexanes, followed by pure ether, as eluents. Each of the fivecomponents was examined by ¹ H NMR spectroscopy. The third and fourthcomponents (in order of elution from the column) were tentativelyassigned as4-[5-[1,2-dioxo-3-butoxycyclobut-3-en-4-yl]pent-1-yl]-3-butoxycyclobut-3-ene-1,2-dione(0.69 g) and2,3-dibutoxy-4-[5-[1,2-dioxo-3-butoxycyclobut-3-en-4-yl]pent-1-yl]-4-hydroxycyclobut-2-en-1-one(2.14 g).

A portion of the isolated fourth component (2.01 g) was dissolved in THF(20 mL), and the resultant solution was treated with 6M hydrochloricacid (20 mL). The two-phase mixture became warm, and after 15 minutesstirring was observed to have become homogeneous. After a further twohours stirring, the solution was concentrated to dryness under reducedpressure. Water (20 mL) was added, and removed by evaporation, in orderto drive off excess hydrogen chloride. The remaining water was removedby azeotropic distillation under reduced pressure withdichloromethane/acetone, to yield an off-white solid. This material waspurified by recrystallization from THF/ether to yield the desiredcompound as a tan powder (542 mg, 18% yield over two steps). Thestructure of this compound was confirmed by ¹ H and ¹³ C NMRspectroscopy.

Example 11

Preparation of4-[5-[1,2-dioxo-3-[4-methyl-benzyloxy]cyclobut-3-en-4-yl]pent-1-yl]-3-[4-methylbenzyloxy]cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of a dimeric squaric acidderivative in which two [4-methylbenzyloxy]cyclobut-3-ene-1,2-dionegroups are linked via a pentamethylene chain.

Triethylamine (423 rag, 4.18 mmole) and p-methylbenzyl bromide (1.47 g,7.96 mmole) were added sequentially to a suspension of4-[5-[1,2-dioxo-3-hydroxycyclobut-3-en-4-yl]pent-1-yl]-3-hydroxy-cyclobut-3-ene-1,2-dione(526 mg, 2.0 mmole, prepared in Example 10 above) in chloroform (15 mL)at room temperature, and the mixture was then heated at reflux for 9hours. The solvent was removed under reduced pressure, and the resultantoil was purified by flash chromatography on silica gel withdichloromethane, followed by ether, as eluents. The product eluted withether, and was obtained as a yellow oil (591 mg, 63% yield). Thestructure of this compound was confirmed by ¹ H and ¹³ C NMRspectroscopy.

Examples 12-32

Preparation of oxalic acid derivative secondary acid generators

Example 12

Preparation of bis(2-methyl-2-hexyl) oxalate

To a solution of 2-methylhexan-2-ol (4.65 g, 40 mmole) and pyridine(4.74 g, 60 mmole) in tetrahydrofuran (15 mL) was added dropwise at5°-10° C. over a period of 15 minutes a solution of oxalyl chloride(2.54 g, 20 mmole) in THF (6 mL). The resultant suspension was stirredat 20° C. overnight, then diluted with cold water (100 mL) and extractedwith diethyl ether (65 mL). The organic layer was washed with colddilute sulfuric acid, then with aqueous sodium bicarbonate, and finallywith aqueous sodium chloride, then dried over sodium sulfate andevaporated to give the desired product as a pale yellow oil (3.25 g, 62%yield). An analytical sample was obtained by column chromatography onsilica gel with 7% ethyl acetate in hexanes as eluent. The structure ofthis compound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 13

Preparation of bis(α, α-dimethylbenzyl) oxalate

To a solution of α, α-dimethylbenzyl alcohol (5.44 g, 40 mmole) andpyridine (4.74 g) in THF (20 mL) was added dropwise at 5°-10° C. withstirring over a period of 25 minutes a solution of oxalyl chloride (2.54g, 20 mmole) in THF (5 mL). The resultant suspension was stirred at 20°C. for 5 hrs, then poured into 140 mL of 0.5N sulfuric acid kept at 0°C. The oily product which separated was extracted with diethyl ether (60mL) and the ether solution washed with saturated sodium bicarbonate (50mL), and then with saturated aqueous sodium chloride (50 mL). The washedsolution was dried over sodium sulfate and evaporated to give thedesired product as a nearly colorless solid (5.745 g, 88% crude yield).A portion of this product was recrystallized from hexanes to providecolorless needles melting point 76.5°-79° C. The structure of thiscompound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 14

Preparation of bis(p-butoxybenzyl) oxalate

To a solution of p-butoxybenzyl alcohol (1.803 g, 10 mmole) and pyridine(1.185 g, 15 mmole) in dichloromethane (10 mL) was added dropwise over aperiod of 5 minutes a solution of oxalyl chloride (0.635 g, 5 mmole) inmethylene chloride (7 mL) at a temperature of 5°-20° C. The resultantsuspension was stirred at 20° C. overnight, diluted to 50 mL withmethylene chloride, then washed successively with water, dilute sulfuricacid, and aqueous sodium bicarbonate, and finally with brine. The washedsuspension was then dried over sodium sulfate and evaporated to give thedesired product (1.97 g, 76% yield) as colorless plates, melting point113.5°-114.5° C. The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 15

Preparation of bis(α-methylbenzyl) oxalate

To a solution of d,l-α-methylbenzyl alcohol (2.443 g, 20 mmole) andpyridine (2.37 g, 30 mmole) in dichloromethane (20 mL) was added at 5°C. a solution of oxalyl chloride (1.27 g, 10 mmole) in dichloromethane(8 mL). The resultant suspension was stirred at 0° C. for 20 minutes,and then at 20° C. overnight. The suspension was then poured intoice-water and acidified with 1N sulfuric acid (20 mL). The organic layerwas washed with dilute sodium bicarbonate solution, then with brine,dried over sodium sulfate and evaporated to give the desired product asa pale yellow oil (2.661 g, 89% yield). The structure of this compoundwas confirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 16

Preparation of bis(p-methoxy-α-methylbenzyl) oxalate

To a solution of d,l-p-methoxy-α-phenethyl alcohol (3.57 g, 23.4 mmole)in dichloromethane (35 mL) containing 2.78 g (35.8 mmole) of pyridinewas added over a period of 20 minutes at 0° C. a solution of oxalylchloride (1.49 g, 11.8 mmole) in dichloromethane (6 mL). The resultantmixture was stirred at 20° C. for 14 hours, then poured into cold dilutesulfuric acid. The organic layer was washed with cold water, then withdilute sodium bicarbonate, dried over sodium sulfate and evaporated togive the desired product as a colorless oil (4.11 g, 97% yield). A 1.2gram sample of this oil was crystallized from methanol to provide 0.51 gof product as fine matted plates of a mixture of diastereomers meltingat 63°-82° C. The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 17

Preparation of bis(p-methylbenzyl) oxalate

To a solution of p-methylbenzyl alcohol (3.33 g, 27 mmole) in pyridine(7 mL) was added at 0° C. over a period of five minutes oxalyl chloride(0.87 mL, 1.27 g, 10 mmole). The resultant reaction mixture was stirredat 0°-10° C. for one hour, then poured into cold dilute sulfuric acid togive a colorless precipitate, which was collected by filtration andwashed with cold water to give colorless plates. These plates wererecrystallized from methanol and then from hexanes as matted needles.The needles were recrystallized from methanol (30 mL) to provide thedesired product (0.96 g, 32% yield), melting point 100°-100.5° C. Asecond crop of the product (1.20 g, 40% yield) was obtained byconcentration of the mother liquors. The structure of the product wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 18

Preparation of ethyl p-methoxybenzyl oxalate

To a solution of p-methoxybenzyl alcohol (4.49 g, 14.4 mmole) andpyridine (1.92 g, 24.3 mmole) in dichloromethane (10 mL) was added at5°-20° C. a solution of ethyl oxalyl chloride (2.216 g, 16.2 mmole) overa period of 4 minutes. The resultant reaction mixture was stirred at 0°C. for 20 minutes and then at 20° C. overnight. The reaction mixture wasthen poured into ice-water and acidified with 1N sulfuric acid (20 mL).The organic layer was washed with dilute sodium bicarbonate, then withbrine, dried over sodium sulfate and evaporated to give the desiredproduct (3.367 g) as a colorless solid. Recrystallization from hexanesprovided colorless fine irregular prisms, melting point 44°-45° C. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 19

Preparation of2,2-dimethyl-1-[4-methoxybenzyloxalyloxy]prop-3-yl[4-methoxybenzyl]oxalate

A solution of 2,2-dimethylpropane-1,3-diol (24.6 g, 0.236 mole) indichloromethane (200 mL) was added in a slow stream to a solution ofoxalyl chloride (60.0 g, 0.472 mole) in dichloromethane (400 mL) whichhad been precooled to 0° C. using an ice bath, the addition being madeat such a rate that the temperature of the solution did not exceed 10°C. The resultant clear solution was allowed to warm to room temperatureover a period of 30 minutes, and stirred for an additional 30 minutes,then cooled to 0° C. and pyridine (75 g, 0.948 mole) was added, again atsuch a rate as to maintain the temperature of the reaction mixture below10° C. To the resultant yellow suspension was added a solution of4-methoxybenzyl alcohol (65.35 g, 0.473 mole)in dichloromethane (100mL), again keeping the temperature of the reaction mixture to 10° C. orbelow. After the addition had been completed, a cream-coloredprecipitate was observed. The reaction mixture was allowed to warm toroom temperature and stirred overnight.

The mixture was then filtered, and the hygroscopic precipitate ofpyridinium chloride was washed with dichloromethane (2×25 mL). Thecombined organic extracts were washed with: a) water (500 mL) containingconcentrated hydrochloric acid (25 mL); b) water (700 mL) containingsodium hydrogen carbonate (50 g) and c) saturated brine (250 mL). Theorganic layer was then dried over anhydrous sodium sulfate andconcentrated under reduced pressure. The residue was stirred with ether(500 mL) for 10 minutes, then filtered. The precipitate (which was theunwanted by-product, 4-methoxybenzyl oxalate) was washed with more ether(2×25 mL), and the combined ether solutions were concentrated underreduced pressure to give a waxy solid (93.88 g), which resisted attemptsat recrystallization. Purification was, however, effected by triturationwith cold methanol (500 mL) to afford the desired compound (68.5 g, 59%yield) as a white powder, melting point 38°-40° C. The structure of thiscompound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 20

Preparation of2,2-dimethyl-1-[4-benzyloxy[benzyloxalyloxy]]prop-3-yl[4-methoxybenzyl]oxalate

Example 19 was repeated except that the 4-methoxybenzyl alcohol wasreplaced by 4-benzyloxybenzyl alcohol, to give the above compound in 73%yield. This compound had a melting point of 73°-74° C., and itsstructure was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 21

Preparation of 1-[4-methoxybenzyloxalyloxy]]hex-6-yl[4-methoxybenzyl]oxalate

Example 19 was repeated except that the 2,2-dimethylpropane-1,3-diol wasreplaced by hexane-1,6-diol, to give the above compound in 49% yield.This compound had a melting point of 114°-115° C., and its structure wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 22

Preparation of Cyclohexyl[4-[6-[4-[[cyclohexyloxalyloxy]methyl]-phenoxy]hex-6-yloxy]benzyl]oxalate

Part A: Preparation of 4-[1-[4-hydroxymethylphenoxy]hex-6-yloxy]benzylalcohol

4-Hydroxybenzyl alcohol (24.82 g, 0.2 mole) was added to a stirredsuspension of finely ground potassium carbonate (42.0 g, 0.4 mole) indry dimethylformamide (250 mL). The resultant mixture was stirred at 60°C. under dry nitrogen for 10 minutes, then 1,6-dibromohexane (24.4 g,0.1 mole) was added. The reaction mixture was maintained at 60° C. for 5hours, then allowed to cool to room temperature and stirred for 17hours. The reaction mixture was then poured slowly into ice/water (800mL). A tan precipitate formed, which was collected by filtration, washedwith water, and dried in air to give a sticky solid. This material wastriturated with 2-propanol (100 mL) rind then with cold water (200 mL),to give the desired product as a powder (13.8 g, 42% yield) which wascollected by (slow and difficult) filtration. The compound melted at96°-110° C., and its structure was confirmed by mass spectroscopy and by¹ H and ¹³ C NMR spectroscopy.

Part B: Preparation of cyclohexyl[4-[6-[4-[[cyclohexyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

A solution of cyclohexanol (2.0 g, 0.02 mole) in dichloromethane (50 mL)was added over a period of 15 minutes to a solution of oxalyl chloride(2.54 g, 0.02 mole) in dichloromethane (50 mL) cooled on an ice bath.The resultant solution was allowed to warm to room temperature over aperiod of 20 minutes, then stirred for a further 30 minutes, then againcooled, using an ice bath, and pyridine (3.16 g, 0.04 mole) was addedover a two minute period. After 5 minutes standing, solid4-[1-[4-hydroxymethylphenoxy]hex-6-yloxy]benzyl alcohol (prepared inPart A above, 3.30 g, 0.01 mole) was added in portions over a period of15 minutes. The slightly turbid solution which formed was allowed towarm to room temperature and stirred for about 30 hours under nitrogen.This solution was then washed with: a) water (100 mL) containingconcentrated hydrochloric acid (10 mL); b) saturated aqueous sodiumhydrogen carbonate (100 mL) and c) saturated brine (50 mL). The organiclayer was then dried over anhydrous sodium sulfate. Charcoal and Celitewere added, and the solution was then filtered through Celite. Afterconcentration of the filtrate under reduced pressure, the residue waspurified by flash chromatography on silica gel with dichloromethane aseluent, giving the desired compound as a pale yellow oil (0.65 g, 10%yield). The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 23

Preparation of adamantyl[4-[6-[4-[[adamantyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

Example 22, Part B was repeated except that the cyclohexanol wasreplaced by an equimolar amount of adamantanol. The above compound wasproduced as a pale yellow oil in 22% yield, and its structure wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 24

Preparation of Menthyl[4-[6-[4-[[menthyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

Example 22, Part B was repeated except that the cyclohexanol wasreplaced by an equimolar amount of d,l-menthol. The above compound wasproduced as a pale yellow oil in 22% yield, and its structure wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 25

Preparation of 2-methacryloxyethyl p-methoxybenzyl oxalate

Part A: Preparation of 2-methacryloxyethy oxalyl chloride

Oxalyl chloride (50 g) and dichloromethane (50 g) were mixed and cooled,with stirring, in an ice bath to 7°-10° C. To the resultant mixture wasadded 2-hydroxyethyl methacrylate (40 g) over a period of 30 minutes.The resultant mixture was stirred overnight at room temperature under aslow stream of nitrogen, then concentrated on a rotary evaporator forone hour to yield the desired product as a colorless oil (65 g), whichwas sufficiently pure to be used in Part B below without furtherpurification.

Part B: Preparation of 2-methacryloxyethyl p-methoxybenzyl oxalate

p-Methoxybenzyl alcohol (14 g, approximately 0.1 mole) and pyridine (11g, 0.13 mole) were dissolved in dichloromethane (100 mL) and cooled inan ice bath to 2°-4° C. Separately, the product of Part A above (25 g,0.11 mole) was dissolved in dichloromethane (25 mL) and cooled in an icebath. The second solution was added gradually to the first over a periodof 25 minutes while keeping the temperature at 2°-4° C. The resultantreaction mixture was allowed to stand at room temperature overnight,then filtered then filtered through a plug of silica to remove a low Rfimpurity detectable by thin layer chromatography (TLC). Thedichloromethane was then removed by evaporation to yield the desiredproduct as a colorless oil (29 g, 91% yield over two stages). TLC withdichloromethane as eluent gave a single spot, R_(r) 0.45. The structureof the product was confirmed by ¹ H NMR spectroscopy indeuterochloroform, the spectrum being as follows: δ=7.28 (doublet, 2H);6.83 (doublet, 2H); 6.05 (singlet, 1H); 5.50 (singlet, 1H); 5.17(singlet, 2H); 4.43 (triplet, 2H); 4.37 (triplet, 2H); 3.72 (singlet,3H); and 1.86 (singlet, 3H) ppm.

Example 26

Preparation of poly(2-methacryloxyethyl p-methoxybenzyl oxalate)

The product of Example 25 above (29 g) was dissolved in toluene (200 mL)and azobis(isobutyronitrile) (AIBN; 0.3 g) was added. The resultantmixture was heated at 65° C. under nitrogen for 16 hours, additionalAIBN (0.2 g) was added, and the mixture was heated under nitrogen for afurther 24 hours. A polymeric product precipitated as a swollen gel,from which the supernatant liquor was decanted. The gel was washedrepeatedly with diethyl ether; whereupon it deswelled and hardened. Thewashed polymer was dried in vacuo at 40° C. to yield the desired polymer(26 g, approximately 90% yield) as a non-sticky white solid, glasstransition temperature (Tg) 65° C., decomposing at 210° C. in theabsence of any catalyst.

Example 27

Preparation of 4-methacryloxybutyl p-methoxybenzyl oxalate

Example 25 above was repeated, except that 4-hydroxybutyl methacrylatewas substituted for 2-hydroxyethyl methacrylate. The product wasobtained as a colorless oil (yield 85%) and its structure was confirmedby ¹ H NMR spectroscopy in deuterochloroform, the spectrum being asfollows: δ=7.28 (doublet, 2H); 6.83 (doublet, 2H); 6.05 (singlet, 1H);5.50 (singlet, 1H); 5.17 (singlet, 2H); 4.23 (triplet, 2H); 4.13(triplet, 2H); 3.72 (singlet, 3H); 1.86 (singlet, 3H) and 1.72(multipict, 4H) ppm.

Example 28

Preparation of poly(4-methacryloxybutyl p-methoxybenzyl oxalate)

The product of Example 27 above (5 g) was dissolved in toluene (25 mL)and AIBN (0.025 g) was added. The resultant mixture was heated at 65° C.under nitrogen for 16 hours, and then poured into hexane, whereupon thedesired polymeric product precipitated, Tg approximately 50° C.,decomposing above 200° C. in the absence of any catalyst.

Example 29

Preparation of 4-benzyloxybenzyl 2-methacryloxyethyl oxalate

Example 25, Part B above was repeated, except that 4-benzyloxybenzylalcohol was substituted for p-methoxybenzyl alcohol. The product wasobtained as a white solid, melting point 40°-42° C. (yield 85%) and itsstructure was confirmed by ¹ H NMR spectroscopy in deuterochloroform,the spectrum being as follows: δ=7.4 (multipict, 5H); 7.28 (doublet,2H); 6.85 (doublet, 2H); 6.07 (singlet, 1H); 5.52 (singlet, 1H); 5.23(singlet, 2H); 5.02 (singlet, 2H); 4.45 (triplet, 2H); 4.35 (triplet,2H); and 1.88 (singlet, 3H) ppm.

This monomer was converted to its homopolymer in the same manner asdescribed in Example 28 above.

Example 30

Preparation of ethyl 4-(4-vinylbenzyloxy)benzyl oxalate

Part A: Preparation of 4-(4-vinylbenzyloxy)benzyl alcohol

A solution of potassium hydroxide pellets (3.2 g, 0.05 mole) in 50 mL ofethanol was prepared and stirred in a flask under nitrogen. Separately,p-hydroxybenzyl alcohol (6.2 g, 0.05 mole) and p-vinylbenzyl chloride(7.6 g, 0.05 mole) were dissolved in 50 ml of ethanol. The secondsolution was added to the first with stirring under nitrogen, and theresultant mixture was heated to 65° C. overnight. The reaction mixturewas then cooled to room temperature and filtered, and solvent wasremoved from the flitrate on a rotary evaporator to give a tan solid.This solid was extracted with warm water, filtered off and dried,extracted with petroleum ether, filtered off and finally recrystallizedfrom toluene/hexane to yield the desired product as a colorless solid (6g, approximately 50% yield), melting point 110°-112° C. Its structurewas confirmed by ¹ H NMR spectroscopy in deuterochloroform, the spectrumbeing as follows: δ=7.38 (two doublets, J=10 Hz, 4H); 7.23 (doublet, J=10 Hz, 2H); 6.85 (doublet, J=10 Hz, 2H); 6.67 (two doublets, J=10 and 18Hz, 1H); 5.72 (doublet, J=18 Hz, 1H); 5.21 (doublet, J=10 Hz, 1H); 5.0(singlet, 2H); 4.57 (singlet, 2H); and 1.6 (singlet, 1H) ppm.

Part B: Preparation of ethyl 4-(4-vinylbenzyloxy)benzyl oxalate

The product of Part A above (4.8 g, 0.02 mole) and pyridine (2.0 g,0.025 mole) were dissolved in dichloromethane (50 mL) and cooled to10°-12° C. To this solution was added over a period of 10 minutes asolution of ethyloxalyl chloride (3 g, 0.022 mole) in dichloromethane (5mL). TLC of the reaction mixture after the addition had been completedindicated that only a trace of the alcohol starting material remained.The reaction mixture was then filtered through a plug of silica toremove the pyridine salt produced, and the tiltrate was concentrated toproduce the desired produce as white crystals (approximately 90% yield)melting point 93° C. Its structure was confirmed by ¹ H NMR spectroscopyin deuterochloroform, the spectrum being as follows: δ=7.35 (twodoublets, J=10 Hz, 4H); 7.25 (doublet, J=10 Hz, 2H); 6.85 (doublet, J=10Hz, 2H); 6.67 (two doublets, J=10 and 18 Hz, 1H); 5.72 (doublet, J=18Hz, 1H); 5.21 (doublet, J=10 Hz, 1H); 5.18 (singlet, 2H); 5.0 (singlet,2H); 4.27 (quadruplet, J=8 Hz, 2H); and 1.28 (triplet, J=8 Hz, 3H) ppm.

Example 31

Preparation of poly(ethyl 4-(4-vinylbenzyloxy)benzyl oxalate)

The product of Example 30 above (approximately 2 g) was dissolved intoluene (25 mL) and AIBN (0.01 g) was added. The resultant mixture washeated at 65° C. under nitrogen for 16 hours. Proton NMR analysisindicated only about 50% polymerization, so additional AIBN (0.015 g)was added, and the mixture was heated at 65° C. under nitrogen for afurther 16 hours. The resultant slightly viscous solution was pouredinto a 1:1 v/v mixture of diethyl ether and petroleum ether toprecipitate the polymer, which was then treated with petroleum ether fordeswelling. After drying, the desired polymer (approximately 0.7 g) wasobtained as an off-white powder. Proton NMR analysis revealed no traceof remaining monomer.

Example 32

Preparation of 4-(4-vinylbenzyloxy)benzyl oxalate)

3-Phenylpropyloxalyl chloride was prepared by reacting oxalyl chloridewith 3-phenylpropanol in dichloromethane at 10° C. Example 30, Part Bwas then repeated using the 3-phenylpropyloxalyl chloride in place ofethyloxalyl chloride, to produce the product as fine white crystals,melting point 80° C. (81% yield). Its structure was confirmed by ¹ H NMRspectroscopy in deuterochloroform, the spectrum being as follows:δ=7.1-7.4 (multipict, 9H); 7.27 (doublet, 2H); 6.87 (doublet, 2H); 6.67(two doublets, 1H); 5.72 (doublet, 1H); 5.22 (doublet, 1H); 5.20(singlet, 2H); 5.03 (singlet, 2H); 4.21 (triplet, 2H); 2.65 (triplet,2H); and 2.0 (two triplets, 2H) ppm.

Polymerization of this monomer in the same manner as in Example 31 abovegave the corresponding polymer in a yield of 75%. This polymer as foundto give good results as a secondary acid generator.

Imaging and other Processes of the Invention Example 33

Acid-catalyzed decomposition of squaric acid derivatives

This Example demonstrates that, in the presence of 1 mole percentmethanesulfonic acid (a strong acid), various squaric acid derivativesused in the processes and media of the present invention decompose atsubstantially lower temperatures than the same derivatives do in theabsence of this acid, and thus that the thermal breakdown of thesesquaric acid derivatives is catalyzed by strong acids.

Compounds A, B, D, E and F described above were doped with 1 percent byweight methanesulfonic acid (MSA) by addition of the appropriate amountof a 2 mM solution of the acid in dichloromethane, followed byevaporation of the solvent. The acid-treated materials were comparedwith the pure compounds using thermal gravimetric analysis (TGA) under anitrogen atmosphere with a rate of temperature increase of 10° C. perminute. Compounds B and D, whose decompositions did not result in lossof an obvious gaseous by-product, were also examined by differentialscanning calorimetry (DSC) under the same conditions of temperature andinert atmosphere. Calculated onset temperatures of decomposition(COTD's), and percent weight loss (for TGA) or heat evolved (for DSC)are shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                               TGA           DSC                                                      Compound CTOD, °C.                                                                         % loss   CTOD, °C.                                                                      Heat, J/g                                ______________________________________                                        A         90.9      47.5     --      --                                       A + MSA   70.0      46.8     --      --                                       B        199.1      14.4     179.5   503                                      B + MSA  181.4      10.1     166.4   459                                      D        169.4       4.7     156.5   349                                      D + MSA  142.3       3.8     136.2   320                                      E        231.3      62.7     --      --                                       E + MSA  200.6      60.4     --      --                                       F        145.3      29.9     --      --                                       F + MSA  124.9      33.2     --      --                                       ______________________________________                                    

From the data in Table 1, it will be seen that, in the presence of acatalytic amount of the strong acid, Compounds A, B, D, E and Fdecomposed at temperatures lower (by about 15°-30° C.) than the samecompounds decomposed in the absence of the strong acid.

Example 34

Imaging Process of the Invention

This Example illustrates an imaging process of the present invention, inwhich superacid is generated by ultra-violet irradiation of a filmcontaining a superacid precursor (a sulfonium salt), this superacid isused to catalyze the decomposition of a squaric acid derivative, therebyamplifying the quantity of acid present, and the acid forms a visibleimage upon lamination of the film to a second film containing anirreversible indicator dye.

Four coating fluids were prepared as follows:

Fluid A

Triphenylsulfonium hexafluoroarsenate (6 mg) and poly(vinyl butyral)(B-76 Butvar, supplied by Monsanto Chemical Corp., 25 mg), dissolved inmethyl ethyl ketone (0.5 mL)

Fluid B

As Fluid A, with the addition of 10 mg of Compound A

Fluid C

As Fluid A, with the addition of 15 mg of Compound C

Fluid D

A leuco dye of the formula: ##STR7## (20 mg; this leuco dye may preparedby the procedure described in U.S. Pat. No. 4,345,017) and poly(vinylbutyral) (B-76 Butvar, supplied by Monsanto Chemical Corp., 25 mg),dissolved in methyl ethyl ketone (0.5 mL)

These four fluids were coated on to a 4 mil (101 μm) poly(ethyleneterephthalate) film using a number 8 coating rod to produce coatings A-Drespectively.

Coatings A, B and C were then exposed to ultra-violet radiation of 254nm wavelength from a Spectroline Model ENF-240C ultra-violet lamp(available from Spectronics Corporation, Westbury N.Y.) for periods of15, 30, 60 and 120 seconds; one portion of each coating was leftunexposed as a control. One portion of each exposed coating was thenheated to 90° C. for 20 seconds, while a second portion of each exposedcoating was kept at room temperature. Finally, both portions of eachexposed coating were separately laminated, with the coated sides incontact, to portions of Coating D, this lamination being effected at atemperature of 180° F. (88° C.) and 60 psi (0.43 MPa). Lamination causedmixing of the coatings, so that any acid present in the exposed portionsof Coating A, B or C protonated the indicator dye in Coating D andproduced a magenta color, the density of which was proportional to theamount of protonated dye present.

Table 2 below shows the green optical densities produced in each portionof each coating. Coating A, which contains no squaric acid derivative toact as an acid amplifier, is used as a control. For each optical densitymeasured for Coatings B and C, an acid amplification factor ("AAF",i.e., the number of moles of acid liberated from the squaric acidderivative for each mole of superacid which the ultra-violet irradiationliberates from the superacid precursor) was calculated by dividing thedifference in optical density between exposed and unexposed portions ofthe same coating (otherwise treated identically) by the differencebetween exposed and unexposed portions of Coating A treated in the samemanner. The AAF's shown in Table 2 refer to the line coating in theimmediately preceding line of the Table.

                  TABLE 2                                                         ______________________________________                                        Exposure secs.                                                                0            15      30        60    120                                      Coating Green Optical Density                                                 ______________________________________                                        A (no heat)                                                                           0.015    0.053   0.059   0.065 0.094                                  A (heated)                                                                            0.018    0.035   0.036   0.050 0.062                                  B (no heat)                                                                           0.061    0.214   0.425   0.705 0.860                                  AAF     --       4.0     8.3     12.9  10.1                                   B (heated)                                                                            0.028    0.237   0.206   0.232 0.280                                  AAF     --       12.3    9.9     6.4   5.7                                    C (no heat)                                                                           0.027    0.199   0.346   0.909 0.887                                  AAF     --       4.5     7.3     17.6  10.9                                   C (heated)                                                                            0.023    0.712   0.769   0.712 0.848                                  AAF     --       40.5    41.4    19.7  18.8                                   ______________________________________                                    

From Table 2, it will be seen that the use of a squaric acid derivativeprovides very substantial amplification of the acid generated byirradiation of the superacid precursor, with acid amplification factorsin excess of 40 being achieved in some instances.

Example 35

Imaging Process of the Invention

This Example illustrates an imaging process of the present invention, inwhich superacid is generated by ultra-violet irradiation of a two-layerfilm, one layer of which contains a superacid precursor (an iodoniumsalt), this superacid is used to catalyze the decomposition of a squaricacid derivative, thereby amplifying the quantity of acid present, andthe acid forms a visible image upon heating of the film, with consequentmixing of the acid produced with an indicator dye originally present inthe second layer of the film.

Four coating fluids were prepared as follows:

Fluid A

t-Butyl anthracene (5 mg), (4-n-octyloxyphenyl)phenyliodoniumhexafluoroantimonate (10 mg, prepared as per U.S. Pat. No. 4,992,571)and poly(methyl methacrylate) (Elvacite 2021, supplied by DuPont deNemours, Wilmington, Del.; 30 mg), dissolved in methyl ethyl ketone (0.6mL).

Fluid B

As Fluid A, with the addition of 20 mg of Compound A.

Fluid C

As Fluid A, with the addition of 20 mg of Compound H.

Fluid D

As Fluid A, with the addition of 30 mg of the polymer prepared inExample 9 above.

These four fluids were coated on to a 4 mil(101 μm) poly(ethyleneterephthalate) film (ICI Type 3295, supplied by ICI Americas, Inc.,Wilmington, Del.) using a number 18 coating rod to produce coatings A-Drespectively.

To provide the indicator dye layer for the imaging media, an indicatordye coating fluid was prepared as follows. De-ionized water (60 mL) wasadded dropwise to a magnetically stirred solution of a surfactant(Aerosol TR-70, adjusted with potassium hydroxide to pH 6, 0.65 g), theleuco dye used in Example 13 above (2.5 g), a base (HALS-62, supplied byFairmount Chemical Company, Inc., 117 Blanchard Street, Newark, N.J.07105, 0.25 g) and a polymeric binder (Elvacite 2043, supplied by DuPontde Nemours, 2.75 g) in dichloromethane (46 mL). The resultant, veryviscous mixture was sonicated, causing the viscosity to decrease, andthen allowed to stir overnight at room temperature, during which thedichloromethane evaporated. A fluorinated surfactant (FC-120, suppliedby Minnesota Mining and Manufacturing Corporation, St. Paul, Minn., 56mg of a 25% aqueous solution) was then added. A sample of the resultantdispersion (1 mL) was diluted with an equal volume of a 5% aqueoussolution of poly(vinyl alcohol) (Vinol 540). Coatings A-D were thenovercoated with the resultant solution using a number 7 coating rod toform imaging Media A-D respectively.

These Media A-D were then imagewise exposed through their film bases toultra-violet radiation from a Universal UV unit (nominally emitting at375 nm) supplied by Gelman Instrument Company, then heated on a hotplate at 110° C. for 45 seconds. This heating served to causedecomposition of the squaric acid derivatives in regions in which acidhad been generated by ultra-violet irradiation of the sensitizediodonium salt, and to mix the photochemically active layer with theindicator dye layer, thereby producing a visible image whose density wasproportional to the amount of protonated indicator dye present (andtherefore to the amount of acid generated by the iodonium salt anddecomposition of the squaric acid derivatives).

After this heating, the difference in green optical density betweenexposed and unexposed regions were measured for Media B, C and D usingan X-Rite 310 photographic densitometer, supplied by X-Rite, Inc.,Grandville, Mich., with the appropriate filter. The ratio of thisdifference to the difference measured for Medium A, which contained nosquaric acid derivative, was the acid amplification factor (AAF) due tothe squaric acid derivatives. The results are shown in Table 3 below.

                  TABLE 3                                                         ______________________________________                                                      Optical Density                                                 Medium        Difference  AAF                                                 ______________________________________                                        A             0.08        1                                                   B             1.64        20.5                                                C             0.51        6.4                                                 D             0.62        7.8                                                 ______________________________________                                    

From the data in Table 3, it will be seen that the presence of thesquaric acid derivative provided very substantial amplification of theacid produced by ultra-violet irradiation of the superacid precursor.

Example 36

Imaging Process of the Invention using Monomeric Oxalate Derivatives

This Example illustrates an imaging process of the present inventiongenerally similar to that described in Example 35 above, but in whichthe secondary acid generator is a monomeric oxalate derivative.

A series of coatings were prepared as follows:

t-Butyl anthracene (5 rag), (4-n-octyloxyphenyl)phenyliodoniumhexafluoroantimonate (5 mg, prepared as per U.S. Pat. No. 4,992,571) andthe secondary acid generator to be tested (15 mg) were dissolved in a 5%solution of poly(vinyl chloride) (OxyChem 160, supplied by OccidentalChemical Co., 5005 LBJ Freeway, Dallas, Tex. 75244) in 2-butanone (MEK).The resultant solution was coated onto poly(ethylene terephthalate) baseof 4 mil (101 μm) thickness (P4C1A film, available from E. I. DuPont deNemours, Wilmington, Del.) using a number 12 coating rod.

Separately, an indicator dye layer was prepared by coating a solution ofan indicator dye3,3-bis-[1-butyl-2-methyl-1H-indol-3-yl]-1-isobenzofuranone (soldcommercially under the tradename Copikem 20, by Hilton Davis Co., 2235Langdon Farm Road, Cincinnati, Ohio 45237, 1.00 g) and a polymer binderElvacite 2043 (available from E. I. DuPont de Nemours, 1.25 g) in2-butanone (MEK, 25.0 g) at a coverage of 444 mg/ft². (Attentively, anindicator dye layer was prepared by coating an aqueous dispersionsubstantially as described in Example 35 above, using the followingmaterials and coverages:

magenta indicator dye,3,3-bis-[1,2-dimethyl-1H-indol-3-yl]-1-isobenzofuranone (Copikem 3,available from Hilton Davis Co.), 104 mg/ft² ;

hindered amine base, Tinuvin 292 (available from Ciba-Geigy Co.,Ardsdale, N.Y.) 26 mg/ft² ;

surfactant Aerosol TR-70 (supplied by American Cyanamid Co., Wayne, N.J.07470) 13 mg/ft² ; and

polymeric binder Elvacite 2043, 156 mg/ft².

Experiments using this indicator dye layer are marked by an asterisk inTable 4 below.)

The coatings containing the secondary acid generator were exposed,through a step wedge, to ultraviolet radiation from a 1000 W mercuryvapor lamp (filtered to remove wavelengths below about 330 nm) in anuArc 26-1K UV exposure system (available from nuArc company, Inc., 6200W. Howard St., Niles, Ill. 60648). The irradiance at the film plane wasmeasured using a "Light Bug" radiometer, type IL390B, available fromInternational Light, Inc., Newburyport, Mass. 01950. After exposure, thecoating was heated at 65° C. for 20 seconds, then laminated to theindicator dye layer at 250° F. and 60 psig. After lamination, the greendensity (which is proportional to the total amount of acid generated)achieved was measured in an unexposed region and in eight regions, eachof which had received different amounts of ultraviolet exposure.

Table 4 below shows the relationship between UV exposure (in mJ/cm²) andgreen density attained for the various monomeric oxalate derivativesecondary acid generators tested; the column headed "Ex. #" refers tothe Example above in which the relevant secondary acid generator wasprepared, while "None" in this column refers to a control experiment inwhich no secondary acid generator was included.

                  TABLE 4                                                         ______________________________________                                        Green Optical Density After Exposure of (mJ/cm.sup.2)                         Ex.   0      3       6     11    26    42    86                               ______________________________________                                        None  0.04   0.05    0.08  0.10  0.18  0.26  0.42                              12*  0.06   0.07    0.07  0.23  0.51  1.03  1.51                              13*  0.38   0.99    1.41  1.17  1.28  1.49  1.49                              14*  0.07   0.07    0.20  0.28  0.07  1.08  1.12                              15*  0.08   0.10    0.14  0.32  1.00  1.00  1.11                             16    0.08   0.09    0.12  0.26  0.81  0.88  0.99                              18*  0.06   0.06    0.09  0.16  0.76  0.75  0.83                             19    0.05   0.05    0.10  0.36  1.00  0.84  0.92                             20    0.06   0.06    0.07  0.15  0.96  0.96  0.92                             21    0.04   0.05    0.07  0.11  0.43  0.42  0.97                             22    0.08   0.08    0.08  0.09  0.36  0.66  0.80                             23    0.04   0.04    0.07  0.19  0.57  0.60  0.84                             24    0.06   0.13    0.27  0.49  0.78  0.86  0.99                             ______________________________________                                    

From the data in Table 4, it will be seen that, at substantialultraviolet exposures, all of the coatings containing secondary acidgenerators produced substantially higher green optical densities thanthe control coating, thus indicating that the oxalate derivative wasundergoing superacid-catalysed thermal decomposition and "amplifying"the superacid produced by the ultraviolet exposure.

Example 37

Imaging Process of the Invention using Polymeric Oxalate Derivatives

This Example illustrates an imaging process of the present inventiongenerally similar to that described in Example 36 above, but in whichthe secondary acid generator is a polymeric oxalate derivative.

A series of coatings were prepared in substantially the same manner asin Example 36 above, except that the polymeric binder was omitted fromthe test coating. More specifically, t-butyl anthracene (5 mg) and(4-n-octyloxyphenyl)phenyliodonium hexafluoroantimonate (5 mg) and thepolymeric oxalate derivative secondary acid generator being tested (45mg) were dissolved in MEK (0.6 mL), and the resultant solution wascoated, exposed, and laminated, and the green optical density measured,all in the same way as in Example 36 above. The results obtained areshown in Table 5 below.

                  TABLE 5                                                         ______________________________________                                        Green Optical Density After Exposure of (mJ/cm.sup.2)                         Ex. # 0      3      6    11   26   42   86   170  305                         ______________________________________                                        26    0.07   0.07   0.19 0.68 0.99 0.83 0.81 0.82 1.04                         28*  0.13   0.22   0.52 0.92 0.92 1.03 1.04 1.25 1.15                        31    0.05   0.05   0.05 0.07 0.21 0.49 0.55 1.03 0.98                        ______________________________________                                    

From the data in Table 5, it will be seen that, at substantialultraviolet exposures, all of the coatings containing polymericsecondary acid generators produced substantial green optical densities,thus indicating that the polymeric oxalate derivative was undergoingsuperacid-catalysed thermal decomposition and "amplifying" the superacidproduced by the ultraviolet exposure.

From the foregoing, it will be seen that the present invention providesa process for generation of an acid and for forming an image, and animaging medium, which permit generation of a strong acid insubstantially greater quantity (and thus with greater sensitivity) thanby simple irradiation of a superacid precursor.

We claim:
 1. An imaging medium comprising:a superacid precursor capable of generating a superacid upon exposure to actinic radiation; and a secondary acid generator capable of undergoing thermal decomposition to produce a secondary acid, the thermal decomposition of the secondary acid generator being catalyzed by the superacid, the secondary acid generator being selected from the group consisting of:(a) a 3,4-disubstituted-cyclobut-3-ene-1,2-dione in which at least one of the 3- and 4-substituents consists of an oxygen atom bonded to the squaric acid ring, and an alkyl or alkylene group, a partially hydrogenated aryl or arylene group, or an aralkyl group, bonded to said oxygen atom, said 3,4-disubstituted-cyclobut-3-ene-1,2-dione being capable of decomposing so as to cause replacement of the or each original alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkyloxy group of the derivative with a hydroxyl group, thereby producing squaric acid or an acidic squaric acid derivative having one hydroxyl group; and (b) an oxalic acid diester capable of thermal decomposition to form oxalic acid or an oxalic acid monoester having one carboxyl group.
 2. An imaging medium according to claim 1 wherein the superacid precursor comprises a diphenyliodonium compound.
 3. An imaging medium according to claim 2 wherein the superacid precursor comprises a triphenylsulfonium or diphenyliodonium compound.
 4. An imaging medium according to claim 1 wherein the 3,4-disubstituted-cyclobut-3-ene-1,2-dione is selected from the group consisting of:(a) primary and secondary esters of squaric acid in which the α-carbon atom bears a non-basic cation-stabilizing group; (b) tertiary esters of squaric acid in which the α-carbon atom does not have an sp² or sp hybridized carbon atom directly bonded thereto; and (c) tertiary esters of squaric acid in which the α-carbon atom does have an sp² or sp hybridized carbon atom directly bonded thereto, provided that this sp² or sp hybridized carbon atom, or at least one of these sp² or sp hybridized carbon atoms, if more than one such atom is bonded directly to the α-carbon atom, is conjugated with an electron-withdrawing group.
 5. An imaging medium according to claim 1 wherein the squaric acid derivative is of one of the following formulae: ##STR8## in which R¹ is an alkyl group, a partially hydrogenated aromatic group, or an aralkyl group, and R² is a hydrogen atom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino, alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino, dialkylphosphoxy or trialkylsilyl group, subject to the proviso that either or both of the groups R¹ and R² may be attached to a polymer; ##STR9## in which R¹ and R³ independently are each an alkyl group, a partially hydrogenated aryl group or an aralkyl group, subject to the proviso that either or both of the groups R¹ and R³ may be attached to a polymer; and ##STR10## in which n is 0 or 1, and R⁴ is an alkylene group or a partially hydrogenated arylene group;or the squaric acid derivative comprises at least one unit of the formula: ##STR11## in which n is 0 or 1, and R⁵ is an alkylene or partially hydrogenated arylene group.
 6. An imaging medium according to claim 1 wherein the oxalic acid diester is selected from the group consisting of:(a) primary and secondary esters of oxalic acid in which the α-carbon atom bears a non-basic cation-stabilizing group; (b) tertiary esters of oxalic acid in which the α-carbon atom does not have an sp² or sp hybridized carbon atom directly bonded thereto; (c) tertiary esters of oxalic acid in which the α-carbon atom does have an sp² or sp hybridized carbon atom directly bonded thereto, provided that this sp² or sp hybridized carbon atom, or at least one of these sp² or sp hybridized carbon atoms, if more than one such atom is bonded directly to the α-carbon atom, is conjugated with an electron-withdrawing group; (d) an ester formed by condensation of two moles of an alcohol with the bis(hemioxalate) of a diol, provided that the ester contains at least one ester grouping of type (a), (b) or (c); (e) polymeric oxalates derived from polymerization of oxalate esters having an ethylenically unsaturated group, provided that-the ester contains at least one ester grouping of type (a), (b) or (c); and (f) condensation polymers of oxalates, provided that the ester contains at least one ester grouping of type (a), (b) or (c) above.
 7. An imaging medium according to claim 1 wherein the oxalic acid derivative is one which begins to decompose thermally at a temperature in the range of about 140° to about 180° C., as measured by differential scanning calorimetry in a nitrogen atmosphere at a 10° C./minute temperature ramp, in the absence of any catalyst.
 8. An imaging medium according to claim 1 further comprising an acid-sensitive material which changes color in the presence of the secondary acid liberated when the secondary acid generator is decomposed.
 9. An imaging medium according to claim 8 wherein the acid-sensitive material is admixed with an amount of a basic material sufficient to neutralize all the acid which could be liberated by the superacid precursor.
 10. An imaging medium according to claim 8 comprising a first layer or phase containing the superacid precursor and the secondary acid generator and a second layer or phase containing the acid-sensitive material.
 11. An imaging medium according to claim 1 wherein the superacid precursor and the secondary acid generator are under essentially anhydrous conditions.
 12. An imaging medium according to claim 1 further comprising a polymeric binder in which the superacid precursor and the secondary acid generator are dispersed. 